如何在Elm 0.13中使用字段 [英] How to use Fields in Elm 0.13

问题描述

我一直在努力使这些领域发挥作用，但一直失败。我也一直在尝试寻找示例，但我能找到的唯一示例是使用Elm 0.14，该示例使用的新Channel API在Elm 0.13中不可用。

I have been trying to get the fields to work, but keep failing. I have also been trying to look for examples, but the only examples I could find are using Elm 0.14, which use the new Channel API which isn't available in Elm 0.13.

因此，我从中提供的示例开始目录

import Graphics.Input.Field (..)
import Graphics.Input (..)

name : Input Content
name = input noContent

nameField : Signal Element
nameField = field defaultStyle name.handle identity "Name" <~ name.signal

为了使用该字段，我尝试了

And in order to use the field I tried

main : Signal Element
main = Signal.lift2 display Window.dimensions gameState

display : (Int,Int) -> GameState -> Element
display (w,h) g =
    container w h middle <|
        collage gameWidth gameHeight
            (if  | g.state == Menu ->
                    [ rect gameWidth gameHeight
                        |> filled black
                    , toForm nameField
                    , plainText "*The name entered in the nameField*"
                    ]
                | otherwise -> []
            )

但是我一直收到以下错误

But I keep getting the following error

Expected Type: Signal.Signal Graphics.Element.Element
Actual Type: Graphics.Element.Element

为什么不该元素不再是信号...函数定义明确指出它应该输出信号，对吧？现在如何输入名称，然后可以在变量内使用？

Why isn't the element a signal anymore... The function definition clearly states it should output a signal, right? Now how would I be able to enter a name, that I would then be able to use inside a variable?

推荐答案

Elm 0.13有一些令人讨厌的类型错误消息。通常将预期/实际交换。在这种情况下，问题出在 display：（Int，Int）->中使用 nameField：Signal Element 。 GameState->元素。 display 是纯（非信号）功能，但要纯正，您不能在其中的任何地方使用信号。为了解决这个问题，将 nameField 信号提升一个水平，到 main 。要使用在该字段中输入的内容，请使用输入信号：

Elm 0.13 had some annoyingly confusing type error messages. Expected/Actual are usually swapped. In this case the problem comes from using nameField : Signal Element in display : (Int,Int) -> GameState -> Element. display is a pure (non-signal) function, but to be pure, you can't use a signal anywhere in there. To solve this, hoist the nameField signal up a level, to main. To use what is entered in the field, use the input signal:

main : Signal Element
main = Signal.lift4 display Window.dimensions gameState name.signal

nameField : Content -> Element
nameField = field defaultStyle name.handle identity "Name"

display : (Int,Int) -> GameState -> Content -> Element
display (w,h) g currentContent =
    container w h middle <|
        collage gameWidth gameHeight
            (if  | g.state == Menu ->
                    [ rect gameWidth gameHeight
                        |> filled black
                    , toForm (nameField currentContent) -- use something other than `currentContent` here to influence the field content. 
                    , plainText currentContent.string
                    ]
                | otherwise -> []
            )

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